In recent years there has been growing demand for lithium ion cells, nickel hydrogen cells, and other secondary cells for use as power supplies installed in vehicles, or as power supplies for personal computers and portable terminals. In particular, lithium ion cells, which are lightweight and offer high energy density, hold promise for favorable use as high-output power supplies installed in vehicles. With this type of cell, a known cell structure comprises a sheet-form positive electrode (hereinafter also referred to as a positive electrode sheet) and a sheet-form negative electrode (hereinafter also referred to as a negative electrode sheet) are laminated along with a sheet-form separator and wound into a wound electrode unit. For example, Japanese Patent Application Laid-Open No. 2003-249423 (Patent Document 1) discloses a cell comprising a wound electrode unit with a flat shape in which a positive electrode sheet and a negative electrode sheet are wound via a separator.
Patent Document 1: Japanese Patent Application Laid-Open No. 2003-249423
With this type of cell, however, the flat electrode unit and the cell case have to be manufactured separately, and the electrode unit then housed in the cell case. The cell case is usually made from metal since metals have higher physical strength, and in this case, a step in which the electrode unit is encased in an outer covering (such as an insulating film) is typically carried out to insulate the metal case from the electrode unit. As an example of a cell manufacturing processing that includes this encasing step, a cell is constructed as follows. First, a flat electrode unit is housed (encased) in a flat, box-shaped outer covering that corresponds to the shape of the electrode unit, and then this outer covering is housed along with the electrode unit in a cell case, after which the opening at the upper end of the cell case is closed off with a lid to construct a cell.
However, with a conventional cell constitution, since the outer covering is in the form of a box having corners, it was difficult to insert the outer covering (and the electrode unit) into the cell case. For example, if the corners (the four corners) in a plan view of the cell case had a rounded shape (R-shape), the corners of the outer covering interfered with the corners of the cell case, making it difficult to insert the electrode unit housed in the outer covering. Also, with an electrode unit constituted in this way, during insertion into the cell case, in some cases there was the risk of wrinkling, bending (buckling), tearing, or the like in the outer covering. If this should happen, the insulating function of the outer covering is impaired, and there is the risk that insulation between the electrode unit and the cell case will be lost and an internal short will occur, so wrinkling and so forth of the outer covering needs to be prevented.